I believe my Builders geyser is faulty

I bought a builders geyser 150L at the kempton park branch ( Pretoria road) I left it at the store for a couple of months as I was still building when the time was right I went to collect it . Fitted it but my water was never hot I went to the builders in selcourt and was advised it’s either my thermostat or element but I was surprised that it’s a new geyser that has never worked none the less I replaced my thermostat and even bought a builders brand element but still I’m not convinced my water is not convincing I can’t stand less than 10 minutes in a shower with a geyser with a capacity of 150 liters and the water just becomes cold . Something does not add up . Please intervene I’m not happy with this builders geyser